The present invention relates to a method and apparatus for monitoring a subject, for instance in a seat of a land, sea or air based vehicle.
It is desirable to be able to monitor the physiological state of a subject in a vehicle seat. One approach is to attach adhesive electrodes to the skin of the subject to acquire a cardiac signal. However this invasive method is cumbersome and not suitable for many applications. It is also desirable to detect the presence or absence of a subject in a vehicle seat. Conventionally this is achieved by installing a crude pressure switch in the vehicle seat. However, such a detector is unable to distinguish between animate and inanimate objects.
A method of extracting cardiac and respiratory signals is used in the infant health monitoring system of U.S. Pat. No. 5,479,932. A signal is acquired from the infant, and the cardiac and respiratory signals are extracted using fixed bandpass filters. A problem with this approach is that the cardiac and respiratory signals may move out of these fixed frequency bands. In addition, there is some overlap in the bandwidth of cardiac and respiratory signals. This results in cardiac signal artefacts in the respiratory signal, and vice versa.
In accordance with the invention there is provided a method of monitoring a human or animal subject comprising:
a) acquiring a signal from the subject with a transducer;
b) bandpass filtering the signal in accordance with a set of cardiac filter coefficients to extract a cardiac signal;
c) bandpass filtering the signal in accordance with a set of respiratory filter coefficients to extract a respiratory signal;
d) adapting the set of cardiac filter coefficients so as to reduce artefacts in the cardiac signal;
e) adapting the set of respiratory filter coefficients so as to reduce artefacts in the respiratory signal; and
f) outputting the cardiac and respiratory signals.
By adaptively filtering both signals, signal artefacts can be reduced. If the signals changed in an uncorrelated way then there would be a problem of overlap between the bandpass regions of the cardiac and respiratory filter coefficients. However, we have recognised that the cardiac and respiratory signals are correlatedxe2x80x94ie. heart rate tends to increase and decrease in tandem with respiratory rate. As a result the cardiac and respiratory filter coefficients also tend to increase and decrease in tandem, and thus in general will not overlap.
The coefficients may be adapted by means of a self-adaption feedback loop which identifies and removes periodic signal artefacts. In a preferred example this is implemented by autocorrelating the associated signal, and adapting the set of coefficients in accordance with the autocorrelation signal. Alternatively, or in addition, arfefacts may be removed by means of a cross-adaption feedback loop. In a preferred example this is implemented by subtracting a scaled version of the respiratory signal from the cardiac signal, or vice versa. Both methods (ie. self adaptive and cross-adaptive feedback) enable artefacts associated with the other signal to be at least partially removed, resulting in more accurate cardiac/respiratory signals.
The invention may be employed in a variety of medical or veterinary applications, including monitoring of an animal""s vital signs during an operation, or infant apnoea monitoring. Alternatively the method may be employed in a vehicle (ie the subject may be monitored in a vehicle).
The transducer can non-invasively acquire a signal from the subject, even if the subject is wearing heavy clothing as might be required in cold winter climates. By extracting cardiac and respiratory signals it is possible to distinguish between an inanimate object (eg a bag) and a human subject.
The cardiac and respiratory signals may be extracted from separate respective signals acquired using two separate transducers. Preferably the cardiac and respiratory signals are both extracted from a common signal. This enables a single transducer to be used.
When used for in-vehicle monitoring, the output cardiac and/or respiratory signal can be transmitted to a variety of systems associated with the vehicle. At the simplest level the cardiac and/or respiratory signal (or a parameter obtained therefrom, such as breathing rate or heart rate) can be displayed to the subject or another person in the vehicle. Alternatively the signal(s) may simply be stored for later analysis, or transmitted via a wireless link to a remote site for real time analysis, as may be useful in order to analyse the performance of a racing driver or fighter pilot.
In a further alternative the method further comprises analysing the cardiac and/or respiratory signal to obtain a parameter indicative of a physiological state of the subject. For instance an alarm (visual or preferably audible) may be generated if the subject enters a drowsy sleep state (indicated by low cardiac/respiratory rates, or other parameters), or if the subject""s cardiac/respiratory rates become abnormal in any other way (eg. high or irregular rates associated with aggressive xe2x80x9croad ragexe2x80x9d behaviour). Alternatively a control device associated with the vehicle""s drive control mechanism (such as a speed limiting mechanism or braking system) may be activated or deactivated when the parameter crosses a predetermined threshold. It is likely that either of these actions will also be enough to wake a sleeping driver.
Alternatively the cardiac and/or respiratory signals may be analysed to determine the presence or absence of a subject in the seat. The seat occupancy determination can then be used to activate or deactivate a variety of systems in the vehicle. For instance if a subject is detected as being present then a visual or audible alarm may be activated, instructing the subject to fasten their seatbelt. Alternatively if no cardiac and/or respiratory signals are present (indicating a lack of seat occupant) then an airbag mechanism may be disabled.
Temperature and/or motor movement signals may also be extracted from the signal so as to provide further information on the subject.
The transducer for acquiring the signal may comprise one or more coaxial cables which utilise a piezoelectric polymer, such as polyvinylidene fluoride (PVDF), as the dielectric material between the centre conductor and the shield. A number of these piezoelectric coaxial cables can be threaded through the padding material of the seat to achieve a matrix of coverage with enough mechanical target detection to allow physical target, cardiac and/or respiratory signals to be reproduced in the coaxial cable, as well as a significant noise input to be extracted. However in a preferred embodiment the transducer comprises a flexible sheet of piezoelectric material. This can cover a wide area of the vehicle seat and thus enables a wide range of sizes of subject to be monitored.
The vehicle may comprise any conveyance for transporting people via land, sea or air. However in a preferred case the vehicle comprises a land based vehicle such as a car, truck etc.
Typically ambient noise is removed by acquiring a noise signal from the subject""s environment, acquiring a target signal from the subject, and subtracting the noise signal from the target signal. This is particularly important in the noisy environment usually associated with vehicles. Typically this is achieved using a pair of differentially connected transducers and an instrumentation amplifier.
The invention also extends to apparatus for carrying out the method. Typically the target signal is acquired with a piezoelectric transducer. The signal processing may be carried out in the analog or digital domain. In a preferred example the adaptive bandpass filtering is implemented by a digital signal processor (DSP).